XXVI Encontro Nacional de Tratamento de Minérios e Metalurgia Extrativa Poços de Caldas-MG, 18 a 22 de Outubro 2015

CHARACTERIZATION OF MINERALS CARRIERS FLUORINE AND CHLORINE ASSOCIATED WITH COPPER ORE

FREITAS, M.E., GRAVINA, E.G.1, PEREIRA, A.C.2, MACHADO, G.A.A.2, TEODORO, M.A.M.2 1,2CIT SENAI FIEMG, ISI em Processamento Mineral, e-mail: [email protected]

ABSTRACT

The chemical composition reflects the formation requirements of a mineral deposit. When combined with the detailed study of rock mineralogy and their textural relationships, it may provide a basis for understanding the form of occurrence of a contaminant element and guide the mineral processing requirements. Often, the high F- content is attributed to the presence of fluorite. However, when the amount of this mineral is incipient, other minerals become important due to its high concentration in the rock. Copper mineralization host rocks are characterized by the existence of a great variety of minerals, and by the superposition of deformation and alteration events, thus leading to complex textural relationships. The most abundant copper sulphides are considered of secondary formation and occur preferentially in amphibole and biotite cleavages and fractures, which are carriers of F-, or as micro inclusions in later alteration minerals. The occurrence of mixed particles in the flotation concentrate portrays the intergrowth relations of minerals in the rocks. Data from mineral chemistry show that F- content decreases in the alteration minerals of biotite and amphibole, which are the main carriers of this element. Cl-, on the other hand, has its contents preserved, or augmented with the evolution of the alteration.

KEYWORDS: Cu-sulfides; biotite; amphibole; bearing minerals F- and Cl-.

RESUMO

A composição química reflete as condições de formação de um depósito mineral, que combinada ao estudo detalhado da mineralogia e da petrografia, pode proporcionar a compreensão da forma de ocorrência de um elemento contaminante e servir de guia para o processamento mineral. Muitas vezes, o alto teor de F-, contaminante em minério de Cu, está associado à fluorita. No entanto, quando a quantidade desse mineral é incipiente, outros se tornam importantes devido à sua alta concentração. Rochas mineralizadas em Cu são caracterizadas pela variabilidade mineralógica e pela superposição dos eventos de deformação e alteração, conduzindo a relações texturais complexas. Nos minérios estudados, os sulfetos de Cu mais abundantes são considerados de formação secundária e ocorrem preferencialmente em clivagens e fraturas de anfibólio e biotita, que são portadores de F-, ou como microinclusões em minerais de alterações posteriores. A ocorrência de partículas mistas nos concentrados da flotação retrata as relações de intercrescimento dos minerais nas rochas. Os dados de química mineral demonstram que o conteúdo de F- diminui nos minerais de alteração de biotita e anfibólio. Cl-, por outro lado, tem o seu conteúdo preservado, ou aumentado com a evolução da alteração.

PALAVRAS-CHAVE: Cu-sulfetos; biotita; anfibólio; minerais portadores de F- e Cl-. Freitas, M.E.; Gravina, E.G.; Pereira, A.C.; Machado, G.A.A.; Teodoro, M.A.M.

1. INTRODUCTION

The mineralogical variety of mineral deposits is linked to the formation and alteration conditions, and in order to enable an economic concentration of the ore, several stages of change and / or deformation are needed. The interaction of magmatic granitic fluids with mafic host rocks in hydrothermal systems, resulting in the geochemical bimodal character and the presence of an impermeable rock contributes to the fluid conduit. Thus, in some deposits, such as copper related to granites and mafic rocks reactive, there are noted sulfides with very different levels of sulfur in regions of relative geographical proximity.

The cupric-gold deposit considered in this work is located in the Serra de Carajás, Brazil. It´s a rare mineralization model (Choque Fernandez et al., 2005) consisting of significantly deformed and hydrothermalized rocks (andesitics basalts and dacites), in which the original mineralogy was completely replaced. The Cu mineralization occurs as sulphide ores and oxidized ore. The sulphide ore occurs in disseminated forms (predominant), massive sulphide and filling fractures. The host rocks are biotite-magnetite schists, with amphibole (grunerite-cummingtonite and hastingsite subordinates), fayalite and garnet. The mineralized portions consist mainly of bornite, chalcopyrite and chalcocite (subject) as well as varying proportions of molybdenite, covellite, cobaltite, saflorite, niquelite, siegenite, Au, Ag, graphite, ilmenite, hematite, Te-Ag, uraninite and rare earths minerals. Amphibole, garnet, quartz and plagioclase may occur in varying proportions, and along other silicates, form 40% of the rocks. Fluorite, greenalite, minnesotaite, stilpnomelane, apatite, monazite, allanite, siderite, goethite and malachite occur in smaller quantities (Choque Fernandez, 2002).

The high content of F- in rock and ore can be explained by the strong influence of granite signature in the area. The presence of late fluorite indicates the importance of fluids rich in F- and Cl- during hydrothermal alteration (Teixeira et al., 2010; Pollard, 2000). The F- and Cl- ions can replace the OH- in various hydrated silicates, such as mica and amphibole. The presence of F- in garnet is less common, but can occur in special formation conditions (Manning & Bird, 1990; Valey et al., 1983; Gunow et al., 1980). The presence of these ions in clay minerals is also dependent on the composition of the original mineral and on the stability of mineral phases.

2. MATERIALS AND METHODS

In the samples studied were performed granulochemical studies, mineral chemistry and mineralogical characterization in order to identify and understand the behavior of the F- and Cl- bearing minerals in the copper ore.

As the main objective research was the minerals carriers F- and Cl- remaining in the cooper ore flotation concentrate, the study was focused on samples of feeding, concentrate and tailing. Additional studies were performed in ROM samples to the knowledge of the mineralization host rock and the relationship of the possible F- bearing minerals with mineral ore. In this sense, granulochemical studies have not been conducted in these samples.

XXVI Encontro Nacional de Tratamento de Minérios e Metalurgia Extrativa Poços de Caldas-MG, 18 a 22 de Outubro 2015

The flotation samples were dried at 90°C and sent for particle size analysis by sieving (mesh opening between 75 and 38 µm) and ciclosyzer (in fractions -75 and - 38 µm). Different size fractions were sent for chemical analysis of whole rock by ICP- AES and ICP-MS. The F was analyzed by specific ion, and the Cl by colorimetry.

Mineralogical analyzes were performed by optical microscopy on polished thin sections and X-ray diffraction. The procedure involved an optical microscope coupled with imaging system, Leica DMLP and diffractometer Shimadzu XRD 6000, with CuKα radiation (λ = 1,5418Å), 0.02 pitch and speed 0.50o/min, from 4° to 80° 2θ. For the interpretation of diffraction the Match! 2 software (Putz, 2003-2015) was applied.

Microchemistry analyzes were performed on an electron microprobe JEOL, JXA- 8900R model with four spectrometers WDS-eight crystals. The following standards were used: F (fluorite), Fe (magnetite), Cl (Cl apatite), Na (jadeite), Ba (barium sulfate synthetic), K (sanidine), Si (quartz), Mn (Mn hortonolite), Mg (Mg hortonolite), Ti (rutile), Al (Al2O3), Zn (synthetic ZnO), Ca (synthetic CaF2). Calibration conditions were: 15kV, 20ŋA, with times of 30s to F, Cl for 20s and 10s for the other elements.

3. RESULTS AND DISCUSSION

3.1. Granulochemical analysis

Based on the results of the whole rock chemical analysis of different size fractions of feeding, concentrate and flotation tailings, the grade distributions of SiO2, Al2O3, K2O, Fe2O3, MgO, Na2O, F and Cu were performed. These oxides were chosen because they represent the major constituents of silicates, such as mica and amphibole, which in turn are the potential carriers of F- that is the target of these studies, while copper is the primary constituent of the ore. The Cl- in the chemical analysis of the whole rock appeared below the detection limit in all samples.

The flotation feed has 29% of particles larger than 75 µm, while 48% below 38 µm, 35% of which is silt. The distribution of SiO2 and Al2O3 appears with a trend pattern: about 33% in the thicker fraction (+75 µm), decreased levels in the intermediate, and increased up to 44% of the finer one (-38 µm). K2O, MgO and Na2O have the same tendency of these two species, but the coarsest fraction reaches 38%, which may be indicative of a higher concentration of mica. Fe2O3 appears mainly in the finer fraction. The same was observed for the distribution of F- and Cu.

The flotation concentrate may be classified as superfine: 86% is below 38 µm. The distribution of oxides shows that more than 83% of the content of SiO2, Al2O3, K2O, - Fe2O3 and Cu are found in fractions below 38 µm. In the case of F , MgO and Na2O, 72% are below 38 µm, and 37% are in the fraction below 14.7 µm.

The flotation tailings are characterized by 42% of superfine material (-38 µm) with about 43% of the material above 30.5 µm. A clear correlation of the distribution of SiO2, Al2O3, K2O, MgO and Na2O can be observed in fractions +75 µm and -38 µm: 38-48% and 33-44%, respectively. The finest fraction (-38 µm) is richest in the contents of F-, Fe, about 48%, and Cu with 55%.

Freitas, M.E.; Gravina, E.G.; Pereira, A.C.; Machado, G.A.A.; Teodoro, M.A.M.

3.2. Mineralogical identification

Mineralogical studies by optical microscopy in rock fragments of ROM were instrumental in helping to identify the mineralogy of feed, concentrate and flotation tailings samples, due to the very fine grain size (40-86% of the material below 38 µm), and to the predominance of mixed particles. The studied ROM fragments were classified into four different rock types:

1. The quartz-garnet-biotite schist is characterized by coarse-grained and penetrative foliation defined by biotite crystals. The garnet crystals are marked by poikiloblastic texture with inclusions of quartz, apatite, biotite, magnetite and fluorite and are anastomosing by foliation and the sigmoidal agglomerate of quartz. Their fractures are filled with chlorite, amphibole, magnetite, bornite, chalcocite and chalcopyrite. The biotite is marked by apatite inclusions, partial amendment to chlorite, fractures and cleavage containing magnetite and sulfides. There are two generations of amphibole. The first belongs to cummingtonite-grunerite series with magnetite, bornite and chalcocite inclusions in cleavages and fractures; it is anastomosed by foliation and its edges change to greenalite and minnesotaite. The second generation can be classified as prismatic crystals of hastingsite, and occurs in fractures of garnet, by cutting the biotite or by growing over the alteration minerals of the first generation amphibole (Figure 1a, Appendix 1).

2. The olivine-amphibole-garnet-biotite schist presents primary mineralogy, which consists of olivine, apatite, magnetite and grunerite. The biotite and garnet are associated with dynamic metamorphism, defining the main foliation. The olivine presents a preserved core, grunerite edges, and may be altered to greenalite and minnesotaite, on the external portions, and talc. The recurring mineral fractures are filled with biotite, magnetite, bornite and chalcocite. This rock also presents two types of amphibole: grunerite and hastingsite. The latter has formed later and occurs associated with the alteration minerals. Prismatic crystals of apatite are associated with the mafic minerals primary. The magnetite, presenting two generations, occurs throughout in the rock, and is associated with garnet, biotite, hornblende and sulfides (Figure 1b, Appendix 1).

3. The actinolite-garnet schist is marked by poikiloblastic crystals of garnet by anastomosing matrix consisting of grunerite wrapped in actinolite crystals. The garnet has its fractures filled by amphibole, chlorite, chalcopyrite, bornite and chalcocite, and its edges have random inclusions of actinolite. It can also show cores filled with micro inclusions with a dirtier aspect and cleaner edges, less fractured and euhedral contours, featuring different stages of crystallization. Magnetite, bornite and chalcocite occur mainly in rifts, fractures and contacts of amphibole crystals, preferably having the elongated shape.

4. The arcosean quartzite has granonematoblastic texture, formed by quartz, partially altered feldspar, amphibole and biotite. Magnetite, titanite and fluorite occur on ancillary amounts. The texture is typical of dynamic recrystallization. Distribution is serial and sinuous contacts. The feldspar crystals are rich in opaque inclusions and are oriented in the foliation. The grunerite amphibole alters to hastingsite. Titanite clusters occur elongated in the foliation and associated with amphibole and fluorite, which are also included in mafic minerals. XXVI Encontro Nacional de Tratamento de Minérios e Metalurgia Extrativa Poços de Caldas-MG, 18 a 22 de Outubro 2015

In the flotation feed sample, crystals may occur singly or forming mixed particles (Figure 1c, Appendix 1), with magnetite, chalcopyrite, bornite and chalcocite. In the concentrate, the sulfides occur in the form of free and mixed particles. The mixed particles with sulfides are mostly amphibole (grunerite and hastingsite), biotite and greenalite, all bearing minerals F- (Figure 1d, Appendix 1). In the tailing sample, one can observe the presence of chalcopyrite included in amphibole and greenalite, and bornite and chalcocite associated with amphibole, greenalite, garnet and biotite.

The results of the XRD analysis carried out in the ROM samples and feed, concentrate and tailings flotation are presented in Table 1 below. The XRD patterns are represented at Figure 2, Appendix 1.

Table 1. Minerals found in the samples analyzed by XRD method (Hs - hastingsite; Gru - grunerite; Hbl - hornblende; Fl - fluorite; Fa - fayalite; Grt - garnet; Ap - apatite; Chr - chromite; Qtz - quartz; Zrc - zircon; Gt - goethite; Mt - magnetite; Bt - biotite; Ms - muscovite; Phl - phlogopite; Ann - annite; Vrm - vermiculite; Clay - clay minerals; Chl - chlorite; An - anorthite; Bn - bornite; Cc - chalcocite; Ccp - chalcopyrite; Mfr - magnesioferrite). ROM Flotation Feed Flotation Concentrate Flotation Tailing Hs x Gru x x x x Hbl x x x Fl x x x x Fa x x x Grt x x x x Ap x x Chr x x x Qtz x x x x Zrc x x Gt x Mt x x Bt x x x x Ms x x x x Phl x x x x Ann x x x x Vrm x x Clay x x x x Chl x x An x Bn x x Cc x Ccp x Mfr x

3.3. Microchemical analysis

Fluoride and chloride can be found in the mineral as a main component of the structural formula, or replacing part of the OH-. In the latter case, all the hydrated minerals are potential carriers of F- and Cl-, such as mica and amphibole. The chemical analyses of the samples indicate that the micas are potassic and ferruginous, belonging to the biotite group and contain F- and Cl-, in grades ranging from 0.3% to 1.7%, and 1.4% to 2.3%, respectively.

The analyzed olivine-amphibole-garnet-biotite schist, the most mafic portion of the ROM and preserved rocks, presents biotite with F- contents varying from 1.0% to 1.7%, which is equivalent to 0.58 to 0.94 atoms per formula unit in a position that Freitas, M.E.; Gravina, E.G.; Pereira, A.C.; Machado, G.A.A.; Teodoro, M.A.M.

holds up to 4 F-, Cl- and OH- together. The calculated formula varies from: 2+ (K1,83Na0,06) (Fe 4,40Mg0,71Al0,39Ti0,17) (Si5,72Al2,28) O22 (OH2,70F0,81Cl0,49) to 2+ (K1,54Na0,11) (Fe 4,65Mg0,68Al0,36Ti0,12) (Si5,86Al2,14) O22 (OH3,05F0,54Cl0,41). This variation with loss of K, Al, Mg, F- and Cl-, and enrichment of Fe and OH- refers to the partial change of biotite to chlorite observed in petrographic analysis.

In the biotite crystals present in the analyzed sample of the flotation feed, the F- content is much lower, ranging from 0.3% to 0.9%, and amounts to 0.28 to 0.50 atoms per formula unit. Cl- contents vary from 1.54 to 2.3%, and amounted to 0.46 to 0.68 atoms per formula unit. The calculated formula varies from: 2+ (K1,80Na0,02)(Fe 4,74Mg0,52Al0,34Ti0,15)(Si5,72Al2,28)O22 (OH3,09F0,27Cl0,64) to 2+ (K1,88Na0,02)(Fe 4,23Mg1,26Al0,30Ti0,10)(Si5,64Al2,36)O22 (OH3,10F0,46Cl0,44).

The differences, in this case, may indicate not only the biotite alteration to chlorite, but also that there are variations in the biotite composition in several rock samples, or even reflect the action of weathering where this mica alterates to hydrobiotite.

There are two types of amphibole: ferromagnesian amphibole, named on petrography as grunerite, and calcic amphibole, named hastingsite. In the least altered portions of grunerite, the F- content ranges is about 0.5% and Cl- content ranges from 0.42% to 0.62%. In most altered parts, whenever the composition approaches a greenalite, the tendency is the loss of F- whose content falls 0.1% and 0.35%, and the increase of Cl-, ranging from 0.6% to 1.0%. The hastingsite is of later formation, is generally well preserved, and may contain micro inclusions of copper sulfides. It contains on average 0.4% to 0.7% F-, and 1.5% to 1.6% Cl-. Although the hastingsite proportion in the rock is smaller than the grunerite, in the flotation concentrate the hastingsite is observed frequently.

Although the petrographic characteristics lead to its classification as hastingsite, the 2+ 3+ obtained formula was: (Ca1.48Na0,82K0,10)(Fe 2.71Fe 1.36Mg0,32Mn0.02Al0,55Ti0,03) (Si6,14Al1.86)O22 (OH1.22F0.35Cl0,43), which according to the classification proposed by Hawthorne & Oberti (2007), it is a ferric iron-tschermakite. The only difference is the ratio of Fe3+, which is higher than traditional hastingsite.

4. CONCLUSIONS

The ROM samples indicate varied mineralogy and very complex textural relations. There are no preserved primary minerals, but there can be highlighted olivine, grunerite, magnetite and garnet as the oldest minerals. The biotite is later, and may be associated with the dynamic metamorphism during the main deformation. However, there are changes that can reflect the compositional variations of the original rock, with more mafic portions, in which there is formation of olivine and amphibole, and more silicate portions, predominantly of garnet and biotite, and with quartz.

The sulfides are secondary and always occur closely associated with metamorphic and alteration minerals, filling fractures and cleavages, or as micro inclusions that were encapsulated during the formation and growth of biotite, amphibole and some alteration minerals (chlorite, greenalite, minnesotaite, talc). The microchemistry XXVI Encontro Nacional de Tratamento de Minérios e Metalurgia Extrativa Poços de Caldas-MG, 18 a 22 de Outubro 2015 analyzes show that the biotite and amphiboles contain F- and Cl- in appreciable quantities. These F- and Cl- content become significant when considering the proportion of biotite found in some samples, which may amount to 45% of the total mineral volume. Another important aspect to consider is that biotite is often found in the concentrate flotation, making composite particles with the copper sulfides, and is therefore a major source of fluoride contamination. Fluorite and apatite occur in minute proportions compared to the proportion of biotite and amphibole.

5. ACKNOWLEDGEMENT

The authors would like to thank ITV and the CIT SENAI FIEMG for their valuable the financial and technical support.

6. REFERENCES

Choque Fernandez OJ, Costa ML, Pollmann H, Brandão PRG. Química mineral e relações texturais entre as fases sulfetadas do minério de cobre de Salobo, Carajás (PA): implicações no beneficiamento. Geochimica Brasiliensis 2005; 19; 67-85.

Choque Fernandez OJ. Microquímica e mineralogia de processos do minério de cobre de Salobo, Carajás. [Tese de Doutorado]. Centro de Geociências, Universidade Federal do Pará, Belém; 2002.

Gurnow AJ, Ludingtin S, Munoz JL. Fluorine in micas from Henderson molibidenite deposit, Colorado. Economic Geology 1980; 75; 1127-1137.

Hawthorne FC, Oberti R. Classification of the Amphiboles. Reviews in Mineralogy & Geochemistry 2007; 67; Mineralogical Society of America.

Manning P, Bird R. Fluorian garnets from host rocks of the Skaergard intrusion: Implicationsfor metamorphic fluid composition. American Mineralogist 1990; 75; 859- 873.

Pollard PJ. Evidence of magmatic fluid and metal source for Fe-oxide Cu-Au mineralization. In: Porter, T.M. (ed) Hydrothermal Iron Oxide Copper Gold and Related Deposits: A Global Prspective. Adelaide, Porter Geoscience Consultancy Publishing, 1: 27-41. 2000.

Putz H. MATCH! Phase Identification from Powder Diffraction – version 2.4. [software]. Bonn, Germany 2015.

Teixeira JBG, Lindenmayer ZG, Silva MG. 2010. Depósitos de Óxidos de Fe-Cu-Au de Carajás. In: Brito, R.S.C.; Silva, M.G.; Kuyumjian, R.M. (eds): Modelos de Depósitos de Cobre do Brasil e sua resposta ao intemperismo. pp: 15-48.

Valley JW, Essene EJ, Peacor DR. Fluorine-bearing garnets in Adirondak calc- silicates. American Mineralogist 1983; 68; 441-448. Freitas, M.E.; Gravina, E.G.; Pereira, A.C.; Machado, G.A.A.; Teodoro, M.A.M.

APPENDIX 1: Photomicrographs and XRD patterns of ROM and feed, concentrate and tailings flotation samples.

A B

C D

Figure 1. A) Garnet crystal with large number of fractures filled by the later crystals. LT and PN (range: 200μm). B) Porphyroclast with core olivine with borders altered to amphibole / grunerite and greenalite anastomised by biotite foliation. LT and PN (Range: 1000μm). C) Amphibole crystals (hastingsite), garnet and biotite with inclusions of magnetite, bornite and chalcocite. RL and PN (range: 50μm). D) Free and mixed particles of bornite sulfide, chalcocite and chalcopyrite, the latest associated to garnet, biotite and amphibole. RL and PN (range: 50μm). For legend see Table 1.

Figure 2. A) X-ray pattern of ROM sample. B) X-ray patterns of the flotation tailing, the flotation concentrate and the flotation feed samples together for comparison. For legend see Table 1.